Lab 7: Spectral Classification of Stars
To learn the basic techniques and criteria of the spectral classification sequence by:
Classification lies at the foundation of nearly every science. Scientists develop classification systems based on perceived patterns and relationships. Biologists classify plants and animals into subgroups called genus and species. Geologists have an elaborate system of classification for rocks and minerals. Astronomers are no different. We classify planets according to their composition (terrestrial or Jovian), galaxies according to their shape (spiral, elliptical, or irregular), and stars according to their spectra.
In this exercise you will classify six stars by repeating the process that was developed by the women at Harvard around the turn of the 20th century. The resulting classification was a key step in elucidating the underlying physics that produces stellar spectra. Thus, in astronomy as well as biology, the relatively mundane step of classification eventually yields critical insights that allow us to understand our world.
The spectrum of a star is composed primarily of blackbody radiation–radiation that produces a continuous spectrum (the continuum). The star emits light over the entire electromagnetic spectrum, from the x-ray to the radio. However, stars do not emit the same amount of energy at all wavelengths. The peak emission of their blackbody radiation comes at a wavelength determined by their surface temperature, the relationship known as Wien’s Law. Most stars put out the maximum amount of radiation in and around the optical part of the electromagnetic spectrum.
The diagram at the top shows three blackbody curves for stars of different temperatures. As the temperature drops, the relative flux decreases, and the peak moves from the blue (hot) to the red (cool) wavelength regions of the spectrum.
In addition to the continuous spectrum, a star’s spectrum will feature a number of either emission or absorption lines. Emission lines are produced by atoms when electrons drop from high energy levels to lower ones, emitting photons at specific frequencies in the process. This process adds radiation to the star’s spectrum; emission lines are brighter than the region of the spectrum around them. Absorption lines are produced by atoms when their electrons absorb radiation at a specific frequency, thereby causing the electrons to move from a lower energy level to a higher one. This process removes some of the continuum being produced by the star and results in dark features in the spectrum. These lines are dimmer than the wavelength region around them.
Stars come in a wide range of temperatures. The hottest stars in the sky have temperatures in excess of 40,000 K, whereas the coolest stars that we can detect optically have temperatures on the of 1000-1500 K. The characteristics of the spectrum of a star are strongly dependent on its temperature. Differences in temperatures among stars change the energy output of the star, the wavelength where the continuous spectrum peaks, and the strength of the emission and absorption lines. In this lab you will compare the strength of a hydrogen absorption line in the spectra of 6 stars and determine how the strength of that particular absorption line changes as a star’s temperature changes.
Before we start classifying stars, let’s take a close look at a hydrogen atom. The single electron of a hydrogen atom can occupy only certain energy levels. (Think of energy levels as unequally spaced steps of a ladder, with a huge 1st step and sequentially smaller steps thereafter. The higher up an electron is on the ladder, the more energy it has.) Astronomers use the letter ‘n’ and a number to designate each energy level. The lowest energy level is called the ‘n = 1’ level, the second lowest level ‘n = 2’, the third ‘n = 3’, etc.
Electrons are free to move from one level to another provided they conserve energy while doing so. If an electron moves down from the 2nd energy level to the 1st (n = 2 to n = 1) then the atom conserves energy by emitting a photon of light. The emitted photon has an energy (and corresponding wavelength) equal to the difference between the 2nd and 1st energy levels (i.e., we get an emission line.) An electron can only move up from the 1st to 2nd energy level if it gains the right amount of energy. The electron typically gains this energy if the atom absorbs a photon of light with the correct energy (or wavelength), giving us an absorption line. In this lab we will be dealing exclusively with absorption lines because they are much more common than emission lines in the spectra of stars.
Although hydrogen has only one electron, there are still many different energy-level transitions that electron can make; thus, hydrogen shows many different absorption lines. Physicists classify these lines based on the (low) energy level the electron begins on (before a photon is absorbed). Absorption lines caused by electrons starting in the n = 1 level and ending in any higher level are a part of the Lyman series. Absorption lines caused by electrons starting in the n = 2 level are a part of the Balmer series, and those caused by electrons starting in the n = 3 level are a part of the Paschen series. (Lyman, Balmer and Paschen were physicists who researched these transitions.)
Note: In this lab all wavelengths are given in units of Ångstroms. 1 Ångstrom = 10-10 meters; 1nm = 10 Ångstroms.
Write all answers in the space below each question.
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Panel 1
Panel 2
Panel 3 Panel 4
Panel 5 Panel 6
Table 1
Panel | Strength of Hα lines(# of boxes) |
1 | |
2 | |
3 | |
4 | |
5 | |
6 |
Table 2
Panel #,Strongest Hα line to weakest | Spectral Class (Letter Designation) | |
Strongest | A | |
B | ||
F | ||
G | ||
K | ||
Weakest | O |
5.One can determine the surface temperature of each star (i.e. the temperature of each star’s photosphere) using thefull spectrum in the top graph of each panel and Wien’s Law:
where T is temperature in Kelvin and λpeak is the wavelength in Ångstroms where the blackbody curve peaks.
Determine the surface temperature of each star using the top graph of each panel by following the procedure below:
b)In the second column of Table 3 write down the wavelength (in Ångstroms) where the blackbody continuumin each panel peaks. If the peak is not shown on the graph, then write down a rough estimate of where you think the curveright peak.
c)Use Wien’s Law (formula above) to calculate the surface temperature of each panel’s star. Record your answer in column 3 of Table 3.
Table 3
Panel | Peak Wavelength (Angstroms) | Surface Temperature (K) |
1 | ||
2 | ||
3 | ||
4 | ||
5 | ||
6 |
Therefore, it is more intuitive to classify stars based on their temperature rather than on their Balmer lines alone. Astronomers re ed the classification sequence such that the hottest stars came first, but they retained the letters originally assigned to each star based on their Balmer line strengths.
Table 4
Panel # (Hottest star to coolest) | Corresponding Spectral Class (Letter designated from table 2) |
Compare and contrast the Biblical perspective of the universe and the Astronomical perspective that you have learnt in this course. Does the study of Astronomy reinforce or challenge the Biblical beliefs?
CONGRATULATIONS, YOU JUST OBTAINED THE (NON-ALPHABETICAL) STELLAR CLASSIFICATION SEQUENCE USED BY ASTRONOMERS.
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